1. Field of the Invention
The present invention relates to the field of centrifugal compressors. It concerns a centrifugal compressor with a flow-stabilizing casing, including
(a) an impeller rotatably supported about a machine axis and having a hub with a plurality of impeller vanes on its periphery; PA1 (b) a compressor casing, which encloses the impeller, whose inner casing contour is matched to the outer contour of the impeller vanes and, together with the hub, forms a flow duct extending between an axial inlet and a radial outlet; and PA1 (c) a plurality of elongated stabilizer slots, which extend in the flow direction in the inlet region of the flow duct, penetrate from the inner casing contour into the casing and are distributed over the inner periphery of the casing. PA1 (d) the stabilizer slots are arranged relative to the impeller vanes in such a way that their front end facing toward the axial inlet is located at a predetermined distance downstream of the vane leading edge.
A radial compressor of this type is, for example, known from U.S. Pat. No. 4,212,585 to Swarden et al.
2. Discussion of Background
In centrifugal compressors, the stable operating range is limited, in the case of severe throttling in the direction of small volume flows, by the so-called "surge line". Reliable operation of the compressor is no longer possible beyond this surge line. In addition, very severe mechanical loads occur on all the components due to the unsteady flow and these loads can lead to damage to the compressor or even to its destruction.
For many applications, there is great interest in displacing the surge line of a centrifugal compressor to smaller volume flows in order to increase the stable working range of the compressor. A precondition for this is knowledge of the causes which are decisive for the appearance of a surge line and determine its position.
There are numerous experimental and theoretical investigations of the phenomenon of the collapse of stable flow through the compressor. To a great extent, specialists agree that a recirculation vortex, which forms in the front impeller region when the compressor is throttled, participates to a large extent in the initiation of surging. All the known designs for displacing the surge line therefore attempt to suppress or influence this vortex.
A solution designated as a "casing treatment" is described in U.S. Pat. No. 4,212,585. It is characterized, as shown in the diagrammatic representation of FIG. 1, by a plurality of narrow stabilizer slots 5 in the compressor casing 4. The stabilizer slots 5 begin at the leading edges 6 of the impeller vanes 3 of the vane 2 or even a short distance upstream of them. The slots 5 are inclined in the direction of rotation of the impeller 2 and are, in some cases, arranged obliquely to the machine axis 7. The slots have a rectangular shape in meridional section, the front wall 5a and the rear wall 5c being oriented approximately at right angles to the machine axis 7.
In the known design, the displacement of the surge line occurs mainly in the upper region of the compressor characteristic diagram. In the lower speed range, on the other hand, this form of casing treatment provides no improvement. To the contrary, there may even be a deterioration in the lower speed range. This form of casing treatment is therefore suitable for compressors which are operated at a rotational speed which only fluctuates slightly from the nominal value. It is unsuitable for compressors which operate with a strongly varying rotational speed, such as compressors in exhaust gas turbochargers, because of the stability problems, already mentioned, at low rotational speed.